There are basically three methods of laying pipe in a marine environment: Conventional Lay, Reel-Lay, and J-Lay.
In Conventional Lay, the pipeline is horizontally fabricated on board the vessel by adding individual lengths to the pipeline as it progresses along the deck of the vessel. Near the stern, the assembled pipeline moves along a stinger to begin its descent to the seafloor. The change from a horizontal orientation along the deck of the vessel to a vertical orientation during its descent (overbend) and then its return to a horizontal orientation along the sea floor (sagbend) create strains in the pipe which must not damage the structural integrity of the pipeline. Strains in these regions are typically limited by specification to bending strains in the range of from 0.15 to 0.35 percent with the lower end of this range representing approximately the limit of elastic bending behavior.
The reasons for such bending strain limitations are (1) to prevent the pipeline from bending collapse and (2) to avoid excessive, permanent "residual curvature" from occurring when elastic bending limits are exceeded. For all but the largest diameter pipelines (30-inch and larger) the primary purpose of the above bending strain limitations is to avoid excessive residual curvature. Should these bending strains be exceeded, then the resulting residual curvature must be corrected or reduced to acceptable limits.
As water depth increased, the normal manner of coping with these bending strain limitations was to increase the stinger support length for the overbend region and to apply horizontal axial tension to the pipeline for the sagbend region. Such horizontal tension was created by utilizing the vessel's mooring/anchoring system to "pull" on the pipeline, while the pipeline is coupled to the lay vessel with a gripping device or pipeline tensioner. Alternatively, vessel thrusters were utilized to supply the required horizontal force but the practical and economic aspects of creating and maintaining the necessary thruster forces quickly become prohibitive as water depth increases.
In the Reel-Lay technique, the to-be-laid pipeline is unwound from a reel as the vessel progresses. By initially winding the pipeline in such a manner, the elastic bending strain limitations are greatly exceeded thereby requiring the pipeline to be "straightened" before it can be laid. This is accomplished by reverse bending the pipeline as it moves along the stinger so as to achieve an acceptable in-situ residual curvature. The Reel-Lay technique also employs the application of axial tension to the pipeline as in the Conventional Lay technique to maintain appropriate bending strains after straightening has occurred. By utilizing residual curvature limitations rather than the more conservative bending strain limitations, Reel-Lay achieves pipeline installation with significantly reduced axial tension requirements and stinger support lengths.
In J-Lay, the pipeline is dispatched from the vessel in a vertical orientation rather than from a horizontal orientation as in Conventional Lay and Reel-Lay. Thus, the J-Lay provides for only one vertically oriented work station as compared to the multiple horizontal work stations available with the Conventional Lay technique.
It is thus an object of this invention to apply the bending limitations used in the Reel-Lay technique to the Conventional Lay operation so as to achieve a significantly increased water depth capability for the Conventional Lay technique. Another object of this invention is to provide a method whereby larger diameter pipelines (i.e., greater than 12") can be laid with this method than is currently possible with the Reel-Lay technique or is currently feasible with the Conventional Lay technique. Further objects of this invention are to reduce the stinger length normally required under the Conventional Lay technique and to reduce the amount of horizontal tension that must be applied to the pipeline. These and other objects of this invention will become obvious upon further investigation,